In response to the Internet and multimedia being in widespread use in recent years, a high-speed, high-capacity optical communications system is demanded. Under the circumstances with such a demand, a system with a transmission rate of 40 Gbit/s has been expected to be implemented on the basis of a faster channel transmission rate and multichannel WDM. In an optical communications system, one of the most important parts exerting an influence on the system performance is an avalanche photodiode which converts an optical signal to an electrical signal. The avalanche photodiode (hereafter referred to as APD) is a device utilized as a low-noise optical receiver, which generates carriers (electron and hole) by optical absorption, multiplies the carrier by an avalanche mechanism, and extract the output electrical signal from the carrier current.
An APD operating at the long-wavelength has already been introduced widely to 2.5 Gbit/s and 10 Gbit/s systems, and a device which is applicable to the next-generation system with the rate of 40 Gbit/s is in process of being developed. A hole-injection type APD having an InP avalanche multiplication layer has been commonly adopted as a relatively low-speed APD structure. Recently, an electron-injection type APD, which is superior in terms of operation speed, has been attracting attention.
A long-wavelength APD having an InGaAs optical absorption layer commonly employs an SAM (Separated Absorption and Multiplication) structure, in which an optical absorption layer and an avalanche multiplication layer (avalanche layer) are separated to suppress an increase of dark current induced by a narrow gap semiconductor. For control of the electric field intensity of the optical absorption layer and of the electric field intensity of the avalanche multiplication layer independently, an electric-field control layer and a bandgap graded layer are provided between the optical absorption layer and the avalanche multiplication layer.
FIG. 4 is a cross-sectional view showing a conventional mesa-type APD. The electron-injection type APD shown in FIG. 4 is configured to have an optical absorption layer 46 composed of a depleted InGaAs and an avalanche multiplication layer 43 composed of either InAlAs or InP. To be more specific, an electric-field control layer 44 and a bandgap graded layer 45 are provided between the avalanche multiplication layer 43 and the InGaAs optical absorption layer 46 which both are provided on an n-InP electrode layer 41. An InGaAs electrode layer 47 is formed on the optical absorption layer 46, and an electrode 49 is further formed thereon. An electrode 48 is also provided on the n-InP electrode layer 41. Unlike the hole-injection type APD, a guard ring structure which is formed by using a Zn diffusion technique cannot be adopted; therefore, a typical electron-injection type ADP is only allowed to have a mesa-type device structure as shown in FIG. 4. As a result, a fundamental problem arises in which dark current is increased around the junction periphery of the device.
In order to solve such a problem with dark current, various proposals have been made. For example, a structure in which an InP regrown layer covers the area around the mesa structure of a device has been proposed (refer to nonpatent document 1), and good characteristics have been reported. A structure in which electric field distribution is modulated around the operation region by ion implantation has also been reported (nonpatent document 2). Furthermore, a structure is also proposed in which a buffer layer including an n-type doped partial in-plane region is inserted between an avalanche multiplication layer and an n electrode layer so that the electric field intensity around the mesa becomes lower than that of the inside in operation (patent document 1).
However, even the proposed techniques described above have the following problems still remained to be solved. In the technique proposed in the nonpatent document 1, implanting a mesa structure of a semiconductor by regrowth is generally difficult, since growth behavior varies according to plane index. The technique also has a disadvantage in terms of production cost. The structure proposed in the nonpatent document 2, in which electric field distribution is modulated around the operation region by ion implantation, creates a trap level due to damage caused by implanted ion. Since impurities are non-activated by the trap level, there is a persisting concern about stability of the device. Regarding the structure proposed in the patent document 1, no practical guidance has been disclosed yet for designing the buffer layer including the n-type doped partial region.
As described above, although it is expected that the electron-injection type APD demonstrates superior features to those of the conventional hole-injection type APD, there were problems in terms of an increase of dark current and securing the device life time. A structure has also been proposed in which a mesa is buried in APD by use of semiconductor; however, the problems in difficulty of the production technique and production cost still remain to be solved. Having been made in consideration of the above-described problems, the present invention aims to provide a production technique of an electron-injection type APD without using of a diffusion technique. In addition, the provide APD has to be capable of providing a stable operation with low dark current. A technique regarding a “guard ring structure” is applied to the present invention for the purpose of achieving stable operation of the electron-injection type APD. Detailed explanations will be provided in the following section.
Patent document 1: Japanese Patent Application Laid-open No. 2005-086109
Patent document 2: Japanese Patent Application Laid-open No. 2005-223022
Nonpatent document 1: S. Tanaka, S. Fujisaki, Y. Matsuoka, T. Tsuchiya, S. Tsuji, “10 Gbit/s Avalanche Photodiodes Applicable to Non-Hermetic Receiver Modules,” OFC2003, Vol. 1, MF55, p 67
Nonpatent document 2: Shono, Endo, Inomoto, Watanabe, Makita, Nakata, “10 Gbit/s high-sensitivity planar type APD for small optical receiver module” 2004 Proceedings of the Institute of Electronics, Information, and Communication Engineers General Conference C-4-37, p 365